Preheat the oven to 400°F Remove the small fillet from the underside of each chicken breast and set to one side. Using a sharp knife make a vertical cut down the length of each chicken breast, but not all the way through to form a pocket.
In a small bowl, mix together the Edam, tomatoes, thyme and season well.
Spoon the Edam mixture into each chicken breast pocket. Fold over the flaps of the pocket and cover with the reserved chicken fillets to enclose the filling.
Wrap 1 ½ slices of prosciutto around each breast.
Heat a non-stick frying pan or griddle pan, lay in the chicken and sear for 2 minutes each side until golden. Transfer to the oven and cook for 12-15 minutes or until the chicken is cooked through.
Slice each chicken breast in half on the diagonal and serve straight away with your choice of accompaniment(s).